The rearrangement of Ig and TCR genes is a highly regulated process. Even though the same enzymes perform V(D)J recombination in both lineages, only T cells rearrange TCR gene segments, and only B cells completely rearrange Ig segments. In addition, the pattern in which the gene segments rearrange is precisely ordered during development. The precise mechanisms by which this lineage-specific and temporal control of rearrangement is regulated are unknown, although it has long been held that the control is affected at the level of accessibility of the individual loci to recombination. Recent work has suggested that modulation of chromatin structure is a key factor in controlling accessibility for rearrangement. We have shown that the transcription factor E2A induces high level of recombination of VKI genes in the non-lymphoid cell line BOSC, while interspersed neighboring VKII genes are induced to rearrange at a much lower level. EBF induces rearrangement of Dh3 and Dh4 genes throughout the D locus, while E2A induces rearrangement of only the most J-proximal Dh genes. In the lambda locus, only EBF induces much rearrangement, and predominantly only if the most J-proximal VA gene. A main aim of this application is to address the mechanism by which only part of these various loci are induced to become accessible for rearrangement by these transcription factors. We will determine if the mechanism by which these transcription factors induce specific genes and subregions of Ig loci to undergo rearrangement involves demethylation of DNA, induction of acetylation of histones or other changes in chromatin structure. We will address the hypothesis that individual genes or specific subregions have binding sites for critical transcription factors which can alter the accessibility of the gene(s). We will search for the cis-acting elements which bind the relevant transcription factors and induce the change in accessibility using EMSA and in vivo footprinting. We will mutate these transcription factor-binding sites in cell lines or in mice, and will determine the effect on accessibility for rearrangement and on chromatin structure. We will determine if other transcription factors such as Pax5 or Btk will induce rearrangement of other Ig loci, such as Vh or lambda.